I have a case of PS HO from LTE to 3G for a UE (RLC UM not supported), Handover is failing in preparation phase and reject is coming from RNC. Is it like that since RLC UM not supported, RNC is simply rejecting or LTE MME should simply reject in this type of scenarios.

After a handover from LTE to WCDMA I noticed that there is always "Activate PDP context request" message.
However, in documentation, I found multiple call flows without that message, but with "Update PDP context".
What causes these differences?